How much do you know about your home heating system? It can be handy to understand the basics, so if you’re wondering, how does a furnace work, we’ve got the answers.
How Does a Furnace Work? Why You Need to Know
To get the most out of your furnace—and heating and cooling system in general—it’s best to schedule twice-annual professional HVAC maintenance. Given that, why is it important to know how your furnace works?
- If the furnace or another HVAC component breaks down, you’ll have a good idea whether it’s something you can repair easily or if you need to call an HVAC expert.
- When you’re talking with furnace and HVAC technicians about HVAC repairs, maintenance, or installation, you’ll understand more easily what they’re talking about.
- When it comes time to replace your furnace or HVAC system, having some background knowledge will help you make the best purchase choices.
How a Furnace Works
The furnace is the part of a central heating system that generates heat. Once the heat has been generated, it’s distributed throughout the home via a network of ducts and vents. These are called forced-air heating systems. They’re called forced-air systems because a component called an air-blower forces hot air into the duct network.
The Heating Cycle
Furnaces have a master switch, typically located on a wall near the furnace itself. It usually looks just like a light switch. When the toggle is in the UP position, the furnace is on; when it’s in the DOWN position, the furnace is off. Once your furnace is switched on, the heating cycle is controlled by the system thermostat.
The start of a heating cycle is triggered by the system thermostat. This temperature-activated switch controls when the furnace turns on and off. Once you set the thermostat temperature, it triggers the furnace if the indoor air temperature dips below that temperature.
What happens next depends on whether the furnace is powered by gas or by electricity.
The Heating Cycle: Gas/Fuel Furnaces
Fuel-powered furnaces operate in the same way whether they’re powered by gas, propane, or oil. The only difference is what kind of fuel is used. However, gas-powered furnaces are more widely-used because they’re more cost-effective in most parts of the country.
In a gas-powered furnace, the thermostat trigger actives a furnace component called the gas valve. This valve opens the gas line, allowing gas to enter the furnace burner. The burner is a combustion chamber where gas is burned to generate heat.
Once the cycle is triggered, there are several components that all work together to produce heat.
Control board: Furnace systems have a small electric-powered circuit board that controls the sequence of events in the heating cycle.
On older systems the control board controls the gas valve, igniter, and flame sensor. In newer furnaces there’s an integrated control board that also controls the furnace blower. This allows for variable speed features that improve efficiency.
Pilot light: In older furnace systems the pilot light is a small gas-powered flame that burns constantly, whether the furnace is actively heating or not. The function of the pilot light is to provide the spark needed to start burning gas for heat production.
Thermocouple: This is a safety feature that is paired with a pilot light. The thermocouple is a small rod that heats up when in the presence of a pilot light flame. If the flame goes out, the thermocouple cools down, triggering a shut-off valve to stop gas flow to the pilot light. This prevents gas being emitted and not burned, which might lead to harmful gas build-up in the home.
Igniter: Modern furnaces have an igniter instead of a pilot light.
Gas valve: The gas valve opens to allow gas into the burner, when triggered by the thermostat.
Flame sensor: This is a metallic rod that generates a small electrical current in the presence of a flame.As the gas valve opens, the flame sensor is activated. If there’s no flame, the sensor triggers to shut off the furnace. This is a safety mechanism designed to prevent dangerous natural gas or carbon monoxide leaks.
Heat exchanger: Cool air enters the heat exchanger and is warmed, then exits to be distributed via the duct network. In a gas furnace, the heat exchanger is a series of metal tubes. In an electric furnace, it’s a series of electric elements.
In high-efficiency gas furnaces, heat transfer is improved by adding extra heat exchanges and/or by including heat exchanges with curved surfaces. The curvature slows down air flow across the surface, which allows more time for heat exchange to occur.
Blower fan: This component pulls cold air into the burner and simultaneously directs warmed air into the ductwork. In high-efficiency furnace models, the fan is draft-induced, which makes it burn fuel more efficiently.
How a Gas Furnace Works, Step by Step
- The thermostat detects that the temperature in your home is lower than the set temperature.
- The thermostat triggers the furnace gas valve to open.
- The gas valve opens, allowing gas to enter the furnace burner.
- The flame sensor checks for the presence of a flame. If the pilot light/igniter is working correctly, there will be a flame present inside the burner. If there’s no flame, the sensor will shut down the furnace.
- The blower starts up, drawing air into the burner.
- The pilot light/igniter ignites the gas as it starts to flow through the open valve and into the burner.
- The heat exchanger is heated by the ignited gas.
- Air is warmed as it flows over the heat exchanger.
- Warmed air is forced through the duct network, where it’s distributed throughout the home via the system of ducts and vents.
- Combustion gases, including carbon monoxide, are vented via an exhaust pipe to the outside of the house.
- The system continues to operate, pulling cool air into the burner and forcing warm air into the ducts until the thermostat senses that the indoor air temperature has reached the set point. The thermostat then triggers the furnace to switch off.
- The indoor temperature slowly decreases until it dips below the set point, and then the cycle starts again.
The Heating Cycle: Electric Furnaces
Electricity-powered furnaces are the same as gas furnaces in most respects. The main difference between gas and electric furnaces is that electric furnaces use a slightly different method to generate heat. Otherwise, the processes are the same.
An electric furnace has an igniter, flame sensor, and burner, just like a gas furnace. The difference is that in the burner, an electric furnace has electric heating elements instead of metal tubes, like in a gas heat exchanger. Air is heated in the same way, but the heat is generated via electricity instead of combustion.
Electric furnaces also have some extra components that gas furnaces don’t have. These are a contactor, sequencer, and transformer. These electrical components work in sequence to control voltage to the various electric-powered parts of the furnace. They ensure voltage goes to the right places, in the right order.
Need Help With Your Furnace? CW Service Pros Are Ready for Your Call
Furnaces are complicated systems, with both mechanical and electrical parts. It’s no surprise they need regular maintenance, and perhaps occasional repair, to keep them in good working order. When you need help with your furnace, call CW Service Pros. You can rely on our expertise to keep your furnace in great shape and your home warm all winter.